The choice of words is incredibly (if unintentionally) telling. Like with tablets, Microsoft would rather cram a desktop OS "down" into mobile form, than build off of a simple mobile platform into something more complex.

Updated: Microsoft has clarified to Business Insider that Steve Ballmer did not, in fact, say that Windows 8 is coming to phones (although there appears to still be some question over this). Here's the official transcript from Microsoft: